1. Field of the Invention
The present invention is directed to a heat radiation shield and to a Dewar incorporating such a heat radiation shield therein.
2. Description of the Prior Art
In diagnostic nuclear magnetic resonance low field systems with correspondingly low frequencies of the nuclear magnetic resonance signals, the transmission and reception antennas for the excitation and reception of the nuclear magnetic resonance signals are attenuated only slightly by the body of a patient. There is an increasing number of applications at higher main magnetic field strengths, however, which require a greater distance of the radio-frequency antenna from the patient than was previously the case. This is particularly the case for interventional techniques or intervention using nuclear magnetic resonance monitoring, whereby an operator or physician requires a region of free access to the patient that is as large as possible. Due to the greater distance, attenuation of the antenna due to the patient is also thereby small, so that the losses of the antenna itself play an increasingly important role.
With superconducting coils or antennas, very high quality (Q) values can be reached, on the order of magnitude of over 100,000, i.e., the antenna losses are practical negligible. The use of loss-free antennas of this sort is thus recommended. Care must be taken, however, to ensure that the high quality is also maintained during operation. This requires an effective heat isolation against the heat radiation from the patient. In the field of cryogenics, super-insulating foils are used as a heat shield, which consist of a thin polyester film with a vapor-deposited metal layer made of aluminum. Super-insulating foils of this sort, however, cannot be used in superconducting antennas. The radio-frequency eddy currents induced in the metal layer would drastically reduce the high quality of the coil of antenna.
It is, for example, known from U.S. Pat. No. 4,636,730 to provide an antenna for a nuclear magnetic resonance apparatus with a shielding in order to keep electrical fields produced by the antenna away from the patient (Faraday shield). The shielding, however, must be transparent to form the magnetic components of the radio-frequency field. According to an embodiment disclosed in this patent, the shielding consists of strip conductors connected electrically with one another. An electrical shielding of this sort cannot be used as a heat radiation shield.
From JP-OS 6437813, a superconducting antenna is known for nuclear magnetic resonance apparatuses, which is arranged in a heat-insulating housing.